1. Field of the Invention
The present invention relates to an optical emission analysis apparatus, in which a discharging process is applied to evaporate atoms of a sample for emitting light; and the emission intensity is measured, so as to analyze elements of the sample. More particularly, the present invention relates to an optical emission analysis apparatus, in which a large-current spark discharge is produced between the metal sample and the discharge electrode, and an amount of elements is analyzed simultaneously in a short time.
2. Description of Related Art
In the optical emission analysis apparatus, the spark discharge is produced between the metal sample and the discharge electrode (discharge gap). The large-current discharge is used for evaporating the atoms on the surface of the metal sample, while the discharge plasma is used for exciting the atoms. Since each of the excited atoms and/or ions emits light according to the specific line spectrum of each element, the amount of elements existed in the plasma can be specified by introducing the light into a spectrometer and measuring the light intensity at a specific wavelength. By simultaneously measuring the light intensities with a plurality of wavelengths, the amount of various elements in the plasma can be determined. Thus, the relative amount of elements composing the metal sample can be specified.
In a conventional optical emission analysis apparatus (shown in FIG. 3), a main discharge power supply 12 and an ignitor circuit 13 are connected with a discharge gap 11 formed by a metal sample 32 and an electrode 31, so as to form a main discharge current path. In the main discharge power supply 12, a capacitor is charged to hundreds of volts (V). After the discharge begins in the discharge gap 11 (the gap between the metal sample 32 and the electrode 31), the energy for forming the large-current spark discharge is provided. A controller 14 controls the charging voltage and the timings of the main discharge power supply 12 and the ignitor circuit 13.
In order to avoid change of the surface condition of the sample during the analysis, a rare gas is usually filled in the gap between the metal sample 32 and the electrode 31. The metal sample 32 and the electrode 31 are arranged and spaced apart for approximately several millimeters (mm) to avoid causing a discharge at a voltage of hundreds of volts. The ignitor circuit 13 is used to apply a high voltage of approximately 10 kV, generated on the secondary coil of the ignition transformer 21, to the electrode 31, so as to start the discharge.
An excitation power supply 23 and a current control device 22 are connected to a primary coil of the ignition transformer 21, so as to form an excitation current path. Firstly, by turning on the current control device 22, the current flows from the excitation power supply 23 to the primary coil, so as to excite the primary coil. In the mean time, although the charged voltage in the capacitor of the main discharge power supply 12 has already been applied through a secondary coil of the ignition transformer 21, the discharge does not begin in the discharge gap 11 because the voltage is low.
When a predetermined current flows through the primary coil, by turning off the current control device 22, the secondary coil generates an induced voltage of more than 10 kV due to the magnetic energy accumulated in the ignition transformer 21. The insulation of the discharge gap 11 (the gap between the metal sample 32 and the electrode 31) is thereby destroyed to begin discharge.
Once the discharge begins, the main discharge power supply 12 supplies energy to the discharge gap 11 through the secondary coil of the ignition transformer 21, such that the discharge current rapidly increases, and a high-energy spark discharge occurs in the discharge gap 11. In the mean time, high temperature is formed at a part of the surface of the metal sample 32, such that the atoms of the sample begin to evaporate.
The evaporated atoms are excited by electrons in the plasma. When the excited atoms return to a stable state, the atoms emit lights with specific wavelength corresponding to the energy difference. Since each element has specific energy levels, the wavelengths of the light also form the line spectrum of the element. The emitted lights in the plasma are effectively introduced into the spectrometer, and the light intensities of elements are simultaneously measured. The ratio of light intensity of each wavelength is not solely proportional to the ratio of the corresponding elements. However, since the amount of each element is approximately proportional, by obtaining the relationship between the emission intensity and the amount of elements beforehand, the emission intensity can be converted to the amount of each element, so as to determine the composition of the elements.
However, the discharge conditions in the plasma generated by the spark discharge vary depending upon the surface condition of the sample. Accordingly, during a plurality of discharges, the amount of the evaporated element or the emission intensity is not constant, but varies randomly each time. Therefore, by repeatedly performing measurements for many times, and by integrating or averaging the signals of the emission intensity, the accuracy of the measured values can be improved. Furthermore, when the measurement begins, the emission intensity is not measured but only the discharge (pre-discharge) is performed. The surface conditions of the metal sample 32 and the electrode 31 await to become stable before the analysis is performed so as to improve the accuracy of the measured values.
The surface of the sample is stripped off due to the discharge. Therefore, after a suitable number of measurements has been performed, a fresh portion of the surface of the sample must be used for measurement, or the surface of the sample must be ground again. It should be noted that the discharge condition must be maintained constant as much as possible.
On the other aspect, the atoms evaporated from the surface of the metal sample 32 due to the discharge are attached on the surface of a peripheral insulator or the electrode 31. The part that cannot be seen from the discharge plasma is fabricated as an insulator to assure insulation. The electrode 31 always faces the metal sample 32 or the plasma surface. The deposits evaporated from the sample are attached on the top portion of the electrode 31, changing the discharge condition, and obstructing a normal discharge ultimately. Therefore, while the grinding of the sample is applied, the electrode 31 also needs to be ground. Therefore, aside from the analysis operation, additional maintenance operation is required, and the operation efficiency of the apparatus is reduced.
In the conventional art as disclosed in Patent Document 1, a technology of reversing the direction (polarity) of the main discharge current to eliminate the deposits on the electrode 31 is disclosed, wherein the burden of a grinding operation of the electrode 31 is obviated.
FIG. 4 is a schematic diagram of the optical emission analysis apparatus in the conventional art. In the structure of FIG. 3, the main discharge power supply 12 has a fixed polarity, whereas in the structure of FIG. 4 the main discharge power supply 15 capable of reversing the polarity is used. However, only by means of reversing the polarity of the main discharge power supply 15, a stable discharge cannot be obtained. Accordingly, an assistant discharge gap 24 is disposed in the ignitor circuit, so as to insulate the ignition transformer from the main discharge current path.
The voltage (discharge starting voltage) of the discharge gap 11, when the discharge begins, varies according to the momentary surface condition of the electrode 31 or the metal sample 32. The magnetic energy of the ignition transformer 21 must supply sufficient energy, such that the capacitive load on the secondary side is charged to generate a voltage higher than the discharge starting voltage when the magnetic energy of the ignition transformer 21 is converted to the electrostatic energy. Therefore, in the structure of FIG. 3, when the discharge begins, certain magnetic energy remains in the ignition transformer 21, so as to maintain a current with the same direction as that of the current when the discharge begins. Therefore, when the polarity of the voltage of the main discharge power supply is reversed, the main discharge current in the reverse direction cannot be effectively increased.
In FIG. 4, the ignition transformer 21 and the main discharge power supply 15 are connected in parallel, such that under the circumstance that the direction of the current when the discharge begins is different from that of the main discharge current, the spark discharge also can be stably improved. The ignition transformer 21 and the main discharge power supply 15 are connected in parallel, so the assistant discharge gap 24 is used to perform a DC insulation for the ignition transformer 21 and the main discharge power supply 15 for preventing the capacitor of the main discharge power supply 15 from discharging to the ignition transformer 21. Once a high voltage is generated in the ignition transformer 24, the insulation of the assistant discharge gap 24 is destroyed to apply a high voltage to the electrode 31. Then, the insulation of the discharge gap 11 is destroyed to start discharging. Once the discharge begins and the voltage of the electrode 31 drops, the discharge between the electrodes of the assistant discharge gap 24 is stopped. Only the main discharge current from the main discharge power supply 15 to the discharge gap 11 is increased, and the spark discharge is formed. In the optical emission analysis apparatus shown in FIG. 4, by reversing the polarity of the main discharge current, the deposits on the electrode 31 are eliminated. The burden for the grinding operation of the electrode 31 is thus obviated.
[Patent Document 1] Japanese examined Utility Model Publication No. 56-47564.
In the conventional optical emission analysis apparatus, since a plurality of spark discharges is performed, the deposits evaporated from the sample 32 are formed on the electrode 31. The grinding operation of the electrode 31 to eliminate the deposits becomes a burden. In the conventional art shown in FIG. 4, the deposits on the electrode 31 are eliminated by means of reversing the polarity of discharge, and the grinding operation of the electrode 31 is thereby obviated.
Although the current in the assistant electrode gap 24 is relatively small, the discharge current flows through the assistant electrode gap 24 as the current through the discharge gap 11. So, the electrodes of the assistant electrode gap 24 must also be ground. Although the frequency for performing the grinding operation of the electrode is low, additional maintenance operation aside from the analysis operation is required. Thus, the operation efficiency of the apparatus is reduced.